Heck, that'll make it even easier than I thought.
Logic never goes very far with those consumed in the search for False Dilemmas. America can protect the borders of other countries quite effectively but somehow when it comes to protecting our own borders the task suddenly becomes "too expensive" or "impossible". It's amazing how some peoples' agenda will warp their ability to reason.
Only to Americans suffering from advanced couch potatoitis. If you examine the map, the elevations are from 1,000 meters to 1,500 meters. Not enough to stop anyone from walking through, but more than enough to make construction costs skyrocket.
The Rockies are a far less formidable barrier than the mountains of Afghanistan. And neither the Mujihaddeen or the Taliban ever found the mountains of Central Asia to be any sort of significant barrier to their movement.